Number.NaN

function clean(x) {
  if (x === Number.NaN) {
    // Can never be true
    return null;
  }
  if (isNaN(x)) {
    return 0;
  }
}

console.log(clean(Number.NaN));
// Expected output: 0

function sanitize(x) {
  if (isNaN(x)) {
    return Number.NaN;
  }
  return x;
}